Technical Field
The present invention relates to an alloy powder for overlay welding that is used for performing overlay welding on a steel surface and a weld overlay alloy member and an engine valve obtained using the same. In particular, the present invention relates to an alloy powder for overlay welding, which is preferably used in a high temperature environment, and a weld overlay alloy member and an engine valve that are obtained using the same.
Background Art
Heat-resistant steels have been conventionally used for equipment used in high temperature environments such as inlet air valves and exhaust air valves for internal combustion engines in order to improve wear resistance and the like. In particular, for example, the valve face of an engine valve is required to have properties such as wear resistance, low aggressiveness, heat resistance, and heat shock resistance within a wide temperature range, from the ordinary temperature to high temperatures.
Properties of heat-resistant steels that are generally used as valve materials are insufficient as the aforementioned properties. Therefore, the properties are imparted by melting an alloy powder for overlay welding having the above properties and performing overlay welding (build-up welding) with the molten alloy on a valve face. In particular, an alloy powder for overlay welding (build-up welding) that facilitates formation of an oxide film having excellent oxidation properties is used in an engine for which CNG is used as a fuel because the oxidization power of a combustion atmosphere is weak in such engine.
For example, as an example of such alloy powder for overlay welding, Patent Document 1 suggests an alloy powder for overlay welding containing 0.7% to 1.0% by mass of C, 30% to 40% by mass of Mo, 20% to 30% by mass of Ni, and 10% to 15% by mass of Cr, with the balance made up of incidental impurities and Co in the first embodiment.
At a weld overlay alloy portion (hereafter referred to as “weld overlay alloy”) of a weld overlay alloy member obtained by overlay welding of the above alloy powder for overlay welding, eutectic carbide of Mo and Cr is formed while proeutectic carbide of Mo, which may serve as an origin of fractures, is not formed. In this case, the toughness and wear resistance of a weld overlay alloy can be improved to a greater extent than is possible with conventional weld overlay alloys.
Patent Document 1 further suggests an alloy powder for overlay welding containing 0.2% to 0.5% by mass of C, 30% to 40% by mass of Mo, and 20% to 30% by mass of Ni, with the balance made up of incidental impurities and Co, which is similar to the above alloy powder for overlay welding except that it does not contain Cr, in the second embodiment.
At a weld overlay alloy portion (also referred to as “weld overlay alloy”) of a weld overlay alloy member obtained by overlay welding of the above alloy powder for overlay welding, eutectic carbide of Mo can be formed by setting the content of C to 0.2% to 0.5% by mass relative to the above contents of Mo and Ni while proeutectic carbide that may serve as an origin of fractures is not formed. In this case, the toughness and wear resistance of a weld overlay alloy can be improved to a greater extent than is possible with conventional weld overlay alloys, thereby improving heat shock resistance.